In recent years, with increased demand for high-performance plastics, many polymers of various novel properties have been developed and come onto the market. Especially thermoplastic resins are widely used because of their easy processability, for example, in injection molding. Among them, addition of carbon black for black coloring a product is a generally performed and employed for various components in a wide variety of applications such as automobile components, electric and electronic components, precision machine components and office machine components.
In the case of using a general carbon black, however, the carbon black is likely to produce aggregates, which may cause problems such as reduced strength and insulation failure.
The following resin compositions have been disclosed as the black-colored thermoplastic resin composition. For example, a polyamide composition for color molding has been disclosed (see, e.g., JP S61-55146 A) to contain (i) ethylene bis stearamide and (ii) a carbon black having the particle diameter of 8 to 120 μm in addition to a polyamide resin. A polyphenylene sulfide resin has also been disclosed (see, e.g., JP 2000-230120 A) to contain a carbon black having pH 8 or higher pH in addition to a polyphenylene sulfide resin. These resin compositions, however, have insufficient dispersion of carbon black, which may cause insulation failure in recent thin-wall molded products.
On the other hand, among the thermoplastic resins, liquid crystalline resins that are characterized by the parallel arrangement of molecular chains and show optical anisotropy have been noted because of their excellent flowability, heat resistance, low gas emission and excellent mechanical properties.
The liquid crystalline resin is used as the material suitable for thin-walled parts or electric and electronic components of complicated shapes by taking into account the above characteristics and is employed, for example, connectors, camera modules, relays, switches and coil bobbins. With the recent “light and compact” trend, further reduction of the wall thickness has been strongly demanded in the shape of products. With respect to the above electric and electronic components, further improvements in mechanical strength and flowability have been demanded for the resin composition. On the other hand, there have been the more rigorous quality requirements in production year after year: for example, reducing the percent defective, such as the occurrence of random short in the injection molding process and preventing the resin composition powder from falling off the molding surface in the assembly process. For example, with respect to the connectors, while the wall thickness between the metal terminals of a molded product has further been reduced, a black-colored liquid crystalline resin has been needed for the purpose of simplified discrimination among a plurality of different components to be assembled and easy image analysis for the appearance inspection of the products. With respect to optical components such as camera modules, while a black-colored liquid crystalline resin has been needed from the standpoints of good flowability and light blocking effect, it is strictly demanded to prevent the resin composition powder from falling off the surface of a molded product, which may cause a failure in imaging.
The following resin compositions have additionally been disclosed as the black-colored liquid crystalline resin composition. For example, a liquid crystalline resin composition has been disclosed (see, e.g., JP H07-196894 A) to contain carbon black having pH of 3.5 to 10 relative to 100 parts by weight of a liquid crystalline polyester. A thermoplastic resin composition has also been disclosed (see, e.g., JP H10-101945 A) to contain 0.01 to 10 parts by weight of a carbon black having a dibutyl phthalate (hereinafter referred to as DBP) adsorption amount of not greater than 150 ml/100 g relative to 100 parts by weight of a thermoplastic resin. A liquid crystalline polyester composition has also been disclosed (see, e.g., JP 2001-279066 A) to contain (i) 0.1 to 10 parts by weight of a carbon black having the average particle diameter of 5 to 20 nm and the DBP absorption amount of 60 to 200 cm3/100 g; and (ii) 0 to 180 parts by weight of a fibrous and/or plate-like inorganic filler relative to 100 parts by weight of a liquid crystalline polyester. These resin compositions may, however, produce aggregates of the carbon black and may cause insulation failure between thin-walled metal terminals by the aggregates of the carbon black in molded products such as connectors. The insulation failure is a problem revealed after assembly of products and accordingly has the significant influence on the entire products. An improvement in insulating properties has accordingly been demanded.
The following liquid crystalline resin composition has accordingly been disclosed as the liquid crystalline resin composition having excellent insulating properties, heat resistance and mechanical properties (see, e.g., JP 2009-179763 A). This liquid crystalline resin composition contains: (i) 100 parts by weight of a liquid crystalline resin; (ii) 1 to 10 parts by weight of a carbon black having the primary particle diameter of 10 to 50 nm; and (iii) 0.1 to 10 parts by weight of a talc having the median diameter of 1 to 20 μm. In this liquid crystalline resin, the carbon black is dispersed to have the average particle diameter of not greater than 50 μm.
The above proposed method, however, still causes the presence of carbon black particles having the maximum particle diameter of greater than 50 μm and has difficult in maintaining the insulating properties between recent thinner-walled metal terminals. Production of carbon aggregates may cause a resistance in a thin-wall part of a molded product during resin flow or may cause clogging, which increases the likelihood of a failure such as the occurrence of random short in injection molding. Additionally, the carbon black aggregates may cause the rough surface of a molded product and may cause the resin composition powder to fall off the surface of a molded product, for example, during assembly.
It could therefore be helpful to provide a thermoplastic resin composition having excellent thin-wall flow stability and being capable of producing a molded product that has excellent insulating property even in applications including thin-wall parts that require black color and reduces the resin composition powder falling off the surface of the molded product.